A new mutant of Escherichia coli K-12 supersensitive to both hydrophobic and large hydrophilic antibiotics was isolated and characterized. The mutant grew well at 28 degrees C, poorly at 37 degrees C, and did not grow at 42 degrees C. The rate of its lipid A biosynthesis was reduced as compared to that of the parent strain. This deficiency was rescued by a novel locus, ORF195, the function of which has not been elucidated. ORF195 is located in the 76 min region in the E. coli chromosome and encodes a hypothetical 21.8 kDa protein with no signal sequence. ORF195 isolated from the mutant strain had an identical sequence to the wild-type allele, indicating a suppressor function of the gene product.